Interesting. If you have a really good scope you plan to keep for many years, it would be worth having this done.
On a single mirror (newtonian primary) you could increase transmission 30% over an old aluminium coating. Older 2-mirror scopes - SCT's and Maks - where the coatings last quite well but nevertheless do slowly oxidise with age and after 10 years may may have transmission below 50% - this would restore them to better than new which would be a very noticeable improvement and presumably cheaper than a new OTA.
If it becomes reasonably available here I'd definitely have my Santel Mak done.
On a smallish mirror (star diagonals) with 99% dielectric coating forget it - there isn't any significant improvement to be had.
